Bone marrow spontaneous lesions in rodents from nonclinical 104-week carcinogenicity studies

The authors performed a retrospective study to determine the incidences and range of spontaneous lesions in the bone marrow (sternum and femur) of control mice and rats. Data was collected from 2186 mice (Crl:CD-1(ICR)BR), and 2347 rats (Han Wistar and CD(SD) rats) from the control dose groups of 104-week carcinogenicity studies carried out between 2005 and 2014. The incidence of spontaneous lesions in the bone marrow was higher in mice than in rats, and in both species non-neoplastic lesions were more common than neoplastic lesions. In mice, the most common non-neoplastic lesions in the bone marrow were increased cellularity, pigmented macrophages, and decreased cellularity, and the most common neoplastic lesions were malignant lymphoma, granulocytic leukemia and histiocytic sarcoma. There were occasional sex and site differences (sternum marrow vs femur marrow) in the incidence of a few bone marrow lesions in mice. In rats, the most common non-neoplastic lesions were increased cellularity and stromal fibrosis, and the most common neoplastic lesion was malignant lymphoma. In rats, no sex predilection in the incidence of bone marrow lesions was apparent, and there were no significant site differences in the incidence of lesions. To the best knowledge of the authors, there are no recent reports on spontaneous pathological findings in bone marrow of rodents, and we believe that these results will facilitate the interpretation of background findings and/or their increased incidence in carcinogenicity studies.